Hygienic articles such as disposable diapers, sanitary napkins, incontinence pads and sick bed sheets must have a high absorption capacity to effectively retain eliminated body fluids for acceptable periods of time. Early hygienic articles of this type employed cellulose wadding, fluff cellulose or absorbent cotton as absorbent materials. The problem with these materials is that their moisture-retaining capacity is relatively small compared to their volume. In order to improve the moisture-retaining capacity of hygienic articles made from these materials, the volume of such absorbent materials in the hygienic article must be increased. This produces a bulky product which is unacceptable in many hygienic articles, particularly sanitary napkins.
In an effort to reduce the volume and size of hygienic articles, and increase their absorbent capacity, fluid absorbent substrates have been developed in which highly absorbent materials are combined within the fiber structure of cellulose fluff, wood pulp, textile fibers or other nonwoven fibrous materials. Many substantially water-insoluble absorbent polymers having a high capacity for absorbing water and body fluids have been developed in recent years for enhancing the moisture-absorbent capability of hygienic articles. These polymers are partially or wholly synthetic and are commercially available in fine grain, particulate form or in continuous tows consisting of a number of individual strands. See, for example, U.S. Pat. Nos. 3,997,484; 3,661,815; 4,117,222; and, 3,936,441.
A number of systems have been developed for incorporating a moisture-absorbent material in the nonwoven layer or pad of a hygienic article, such as a disposable diaper, as discussed in U.S. Pat. Nos. 4,927,346 and 5,017,324 owned by the assignee of this invention. In the particular system disclosed in U.S. Pat. Nos. 4,927,346 and 5,017,324, highly moisture-absorbent material in fine grain particulate form is incorporated within the nonwoven pad of a hygienic article within a forming chamber. A perforated conveyor is movable between the inlet and outlet of the forming chamber above a duct which is connected to a source of vacuum. Nonwoven fibers formed of cellulose fluff, wood pulp or other nonwoven materials are introduced into the forming chamber through a conduit and drawn onto the perforated conveyor by operation of the vacuum source. As the nonwoven material is drawn atop the conveyor, one or more spray guns located within the forming chamber intermix the highly moisture-absorbent material with a portion of the fibrous, nonwoven material to form a nonwoven pad having moisture-absorbent material interspersed throughout a predetermined portion of its thickness while leaving other portions of the pad substantially free of highly moisture-absorbent material. In the formation of a disposable diaper, for example, the nonwoven pad containing the moisture-absorbent material is subsequently attached to a thin layer of polyethylene which forms the backing sheet of the finished article.
Currently available hygienic articles, such as disposable diapers, are manufactured with systems of the type described above using highly moisture-absorbent material in fine grain, particulate form. Despite efforts to locate the highly moisture-absorbent particulate material at selected areas within the thickness of a nonwoven pad of such articles, there have nevertheless been problems attributable to the form of the moisture-absorbent material. It has been found that the particles of highly moisture-absorbent material tend to sift through the nonwoven fibers of the nonwoven pad and can be lost through the perforated conveyor within the forming chamber. Particulate, moisture-absorbent material is difficult to contain, even with a filtering system associated with the forming chamber, and environmental contamination can result. Sifting or migration of particulate moisture-absorbent material through the nonwoven fibers is also a problem within the finished hygienic article. It is not uncommon for the moisture-absorbent particles to sift through the nonwoven layer and concentrate at the polyethylene backing sheet of a disposable diaper, for example, producing an undesirable rough, granular feel to the diaper.
Another problem with the use of highly moisture-absorbent material in granular or particulate form within hygienic articles is the speed at which it absorbs urine or other body fluids. While capable of absorbing moisture many times its weight, particles of highly moisture-absorbent material have a relatively small surface area and body fluids must come into contact with such particles in order to be absorbed. Even if a relatively large quantity of particulate moisture-absorbent material is provided within a hygienic article, there are nevertheless spaces or areas within the nonwoven pad with lesser concentrations of particles and therefore a longer time period is required before such moisture or bodily fluids can be absorbed.